Unable to pin a post on my page! Option to do so is missing

On my Pokémon related Facebook page, I have had a pinned post up since March. I want to get rid of that pinned post and pin another. However, the option to pin another post is nonexistent. The option to remove the currently pinned post is also missing. It seems that the old post will be pinned for eternity and I can NEVER change it!

Does anybody know how to fix this?